During angiogenesis, subsets of endothelial cells exhibit unique phenotypes that may be indicative of important changes in cell behavior. Recently, we identified a capillary sprout-specific angiogenic "CD36low" endothelial phenotype that is marked by the downregulation of CD36. Interestingly, CD36 is the receptor through which Thrombospondin-1 (TSP-1) exerts its anti-angiogenic activity. From these basic observations, we generated the hypothesis that the loss of exposure to fluid shear stress elicits the expression of an angiogenic phenotype which, in turn, facilitates continued sprouting. In support of this hypothesis, we have found that the withdrawal of shear stress is a potent inducer of the CD36low phenotype, yielding CD36 expression patterns that are consistent with in vivo observations in sprouts. In addition, we are also interested in identifying and testing other molecules that are regulated by shear stress and critical for sprouting and/or capillary stability. To this end, we have identified angiopoietin-1 (ANG-1), ANG-2, and vascular endothelial growth factor-C (VEGF-C) as potential candidates. Moreover, we have confirmed the in vitro results by showing that ANG-2 is highly expressed at sprout tips. Overall, these results are particularly significant because, to date, no molecular linkages between shear stress and sprouting/stability have been mechanistically verified. This proposal consists of two specific aims. In Aim #1, we will determine whether the application of a "capillary sprouting" shear stress protocol to endothelial cells elicits an angiogenic phenotype that is recapitulated in vivo. Here, we will use a custom RT-PCR array to characterize the "shear-withdrawn" sprouting phenotype and determine whether the in vivo expression patterns of selected phenotypic markers are consistent with regulation by shear stress. This analysis will include CD36, ANG-1, ANG-2, and VEGF-C, which have already been implicated in preliminary studies. In Aim #2, we will determine whether endothelial cells that are pre-conditioned with the same "capillary sprouting" shear stress protocol exhibit enhanced angiogenic function (i.e. proliferation, migration, and barrier function) through changes in the expression of CD36, ANG-1, ANG-2, and/or VEGF-C. If selectively blocking and/or enhancing one or more of these candidate molecules significantly alters angiogenic function, a molecular linkage between shear stress and capillary sprouting/microvessel stability will have been mechanistically isolated for the first time. PUBLIC HEALTH RELEVANCE: During inflammation and tissue injury, new microscopic blood vessels grow by sprouting from existing blood vessels. This process is essential for building functional blood vessel networks in the repaired tissue, and we hypothesize that the withdrawal of exposure to shear stress, which is generated by blood flow, enhances the ability of endothelial cells to make sprouts. In this proposal, we will determine how the removal of shear stress from endothelial cells affects both the expression of proteins associate with sprouting, as well as endothelial cell proliferation, migration, and permeability. In addition, by blocking the function of molecules that are shown to be altered with the removal of shear stress, we will determine which molecules are responsible for eliciting sprouting behaviors.